The attenuation coefficients of each group at different time were calculated from the data of the best exponential fit curve corresponding to the averaged intensity profiles. The selected region is from the depth of 110 µm () to the depth of 280 µm (), where the OCT signal distribution is relatively smooth. Figure 3 presents the attenuation coefficients of normal breast tissue at 0, 5, 15, 30, 45, and 60 min after topical applying 20% G, in which the magenta bar stand for the ultrasound treated group and the yellow bar represent untreated group. Figure 4 shows the changes of the attenuation coefficients in breast cancer tissue treated with 20% G and 20% G/US, respectively. The attenuation coefficients of normal breast tissue are for 20% G and for 20% G/US at 0 min, but it changed to be for 20% G and for 20% G/US at 45 min when the diffusion process reached the stable state. As to breast cancer tissue, the attenuation coefficients are for 20% G and for 20% G/US at 0 min, while at 30 min, it is for 20% G and for 20% G/US. By analyzing these data, it is obvious that the attenuation coefficients in the same kind of breast tissues are almost equal at the beginning no matter whether treated with ultrasound or not (). However, a significant difference in the attenuation coefficients occurs between the groups treated with ultrasound and that without during the agents’ diffusing process (). The attenuation coefficient was reduced by about 52% in normal breast tissue after 45 min treatment of 20% G; while with the treatment of 20% G/US, the attenuation coefficient was decreased by approximately 66%. The similar result was also observed in skin tissue.39 This phenomenon could be induced by the cavitation effect of ultrasound, which has been demonstrated in TDD experiments.34 This effect not only makes biological tissues more permeable but also promotes more hyperosmotic agents penetrating into tissues. Therefore, the refractive indices of the tissue components match better, which reflects in a low attenuation coefficient. Ultrasound has shown the similar effect as a penetration enhancer in the 20% M penetration experiment whose data are not shown. The results have demonstrated that ultrasound has a positive effect on the optical clearing of tissues, and there is a significant difference in the attenuation coefficients between normal breast tissue and tumor tissue ().